1. Field of the Invention
The present invention relates to a high-frequency probe for measuring a high-frequency characteristic by pressing one end against a signal electrode and ground electrodes of a device-under-test and connecting an external measuring instrument to another end to input and output electric signals. In particular, the present invention relates to a high-frequency probe capable of adjusting an impedance characteristic in an end part, the end part being detachable.
2. Description of the Related Art
Conventionally, as this type of high-frequency probe, for example, U.S. Pat. No. 4,849,689 proposes a high-frequency probe that is detachable.
A summary of this high-frequency probe will be described with reference to FIG. 1.
In this high-frequency probe, a probe tip 1010 which contacts to a device-under-test is mutually connected to a connector assembly 1022 for external connection, which is attached to a probe body 1021, through a circuit board 1023. The probe tip 1010 has a central signal line or conductor and ground lines or conductors at both sides thereof on a thin plate-like substrate. The circuit board 1023 also has stripline construction where line-shaped ground lines are provided in both sides of the central signal line with the central conductor as the center.
The circuit board 1023 connecting to the connector assembly 1022 for external connection is fitted in a receptacle 1031 formed as a receiving groove on the upper surface of the probe body 1021 and having the same direction as that of the central signal line. In addition, the circuit board 1023 is fitted with the probe tip 1010 at another end, and consequently, forms a high-frequency transmission line with the central signal and ground lines. Furthermore, the circuit board 1023 is pressed by a pressure block 1040 from the upper direction when the circuit board 1023 is fitted in the receptacle 1031 by an absorber 1024. This absorber 1024 arranges boundary conditions of the transmission line and shields the influence of an external magnetic field.
The pressure block 1040 holds a dielectric compressor bar 1041 and a compression member 1042 and is fitted with stud 1032 of the probe body 1021 using screws. When the circuit board 1023 is pressed by the pressure block 1040, the dielectric compressor bar 1041 not only presses the probe tip 1010 against the circuit board 1023 from the upper direction, but also positions the circuit board 1023. The compression member 1042 is an elastic body to securely press against the probe tip 1010, so as to press the probe tip 1010 against the device-under-test.
The conventional high-frequency probe described above has a problem in that much effort is needed to replace the probe tip, thus requiring many working hours.
The reason for this is that many parts must be disassembled and reassembled, and the parts are small. An example will be described with reference to FIG. 1. First, by removing the pressure block 1040, at this time, the dielectric compressor bar 1041 and the compression member 1042 are removed. Next, by removing the circuit board 1023 and absorber 1024 from the receptacle 1031, the circuit board 1023 and absorber 1024 are separated. Subsequently, the probe tip 1010 fitted with the circuit board 1023 is removed. On the other hand, in assembly, a reverse procedure is performed. In a process like this, careful operation is required because there is a possibility of dropping the dielectric compressor bar 1041, the compression member 1042, and the like around the operation area when they are removed.
Next, an end part 1110 of the probe tip (1010 in FIG. 1) will be described with reference to FIG. 2.
The end part 1110 is an example of an end part of a chip conductor, and has a central signal conductor 1111 and ground conductors 1112 on both sides thereof. These conductors connect to the central signal line and ground lines of the circuit board described above, respectively, and are arranged on the same plane.
As exemplified in FIG. 3A, in a device-under-test 2100 arranged on a surface of a device stage 3000, the height of the signal electrode 2111 provided on the surface as a coplanar electrode is equal to that of the ground electrodes 2112 provided on both sides thereof. On the other hand, in the case shown in FIG. 3B, the height of the central signal electrode 2111B is greater than that of ground electrode 2112 provided on both sides thereof. In this manner, usually, the height of the electrodes arranged in a line is varied.
On the other hand, as shown in FIG. 4A, so as to absorb any height difference between a signal electrode 2111 and ground electrodes 2112 of a device-under-test 2100, only the central signal conductor 1111 in the end part 1110 of the high-frequency probe that is exemplified in FIG. 3A has elasticity. Therefore, in the end part 1110, usually, the central signal conductor 1111, as shown in the drawing, is positioned on the side to be pressed against the device and the central signal conductor 1111 bends with the elasticity according to the height of the signal electrode 2111 when the central signal conductor 1111 contacts to a device-under-test 2100. Therefore, as shown in FIG. 4B, the central signal conductor 1111 contacts the signal electrode 2111 with elastic pressure when the ground conductors 1112 at both sides thereof contact the ground electrodes 2112 of the device-under-test 2100.
In addition, in the high-frequency probe described above, if a ground electrode of the device-under-test is not on the same plane as a signal electrode and is provided instead on the entire surface of the backside of the device, the ground electrodes of the high-frequency probe cannot contact the ground electrodes of the device-under-test. In this case, an alternative method is adopted, wherein the device-under-test is mounted on a board and an end of the high-frequency probe is made to contact measurement electrodes provided on the board.
In addition, conventionally, characteristic impedance of a high-frequency probe is matched in 50.OMEGA. of impedance in a transmission line of the entire probe.
In consequence, the conventional high-frequency probe described above is problematic in that product cost becomes expensive.
This problem arises because the end parts for contacting respective lines of a device-under-test are arranged in a coplanar construction on the same plane; hence, it is necessary to provide ground electrodes adjacent to a signal electrode of the device-under-test within a predetermined space. Thus, this causes the external size of the device-under-test to become large. In particular, in a compound device such as a GaAs whose wafer cost is expensive, the number of chips per wafer becomes small, and therefore cost increase is not avoidable. On the other hand, in the case of the device-under-test that is down-sized by providing the ground electrode on the entire surface of the backside, the measurement electrodes are provided on the board, and the device-under-test is mounted on the board to be measured. Therefore, a defective rate as a product increases and further repair cost is added.
Furthermore, another problem is that matching of the characteristic impedance in the end part of the high-frequency probe collapses and its high-frequency characteristics become worse.
A height difference between the central signal conductor and ground electrodes arises when the conductors of the end part of the high-frequency probe contact the electrodes of the device-under-test. Furthermore, the height of the electrodes of the device-under-test are different and at least the central signal conductor among the conductors in the end part of the high-frequency probe has elasticity.
Thus, the characteristic impedance of the high-frequency probe, as shown in FIG. 3A, is matched when the central signal conductor 1111 and ground conductors 1112 are on the same plane, and the signal electrode 2111 and two ground electrodes 2112 of the device being tested have the same height. Therefore, in the situation shown in FIG. 3A, the characteristic impedance does not fluctuate. Nevertheless, if there are height differences between electrodes as shown in FIG. 3B, matching of the characteristic impedance collapses and the high-frequency characteristic becomes worse since the central signal conductor 1111 and ground conductors 1112 cannot be positioned on the same plane.
In addition, if the probe end is made to be fixed so as to avoid fluctuation of the characteristic impedance, stable contact cannot be achieved since an overdrive amount at the time of contacting and pressing the electrodes of the device-under-test can be obtained little and hence the amount of pressure applied cannot reach a predetermined amount. On the other hand, adjustment of the overdrive amount in which the predetermined pressure amount can be obtained is very difficult, because it is necessary not to damage the electrodes of the device-under-test 2100.
Thus, in the end part 1110 of the high-frequency probe that is shown in FIG. 4A, the contact surface of the electrodes of the device-under-test 2100 is defined as the positioning datum, and hence any height difference between electrodes of the device-under-test 2100 changes the positional relationship between the central signal conductor 1111 and ground conductors 1112. In consequence, matching of the characteristic impedance in the end part collapses, and the high-frequency characteristic becomes worse. Furthermore, in the example shown in FIG. 4B, there is still another problem in that the ground conductors 1112 cannot stably contact the ground electrodes 2112 since the ground conductors 1112 have no elasticity and hence, producibility of measurement is poor.
Furthermore, as shown in FIG. 5A, if a signal electrode 2111 of a device-under-test 2100 is bonded with a wire 2220, characteristic impedance Z0 is obtained by adding an inductive component L1 of the wire 2220 to the characteristic impedance Z1 of the device-under-test 2100 itself. A normal high-frequency probe 1000 shown in FIG. 5B is adjusted to match this characteristic impedance Z0. Therefore, there is an additional problem in that if a single device-under-test that does not have a wire, as shown in FIG. 5B, is tested, it is difficult to set conditions equivalent to the inductive component L1 of the wire, and hence, accuracy of measurement becomes poor.